Throughout this specification, and in the claims, the term “particles” is used in a broad sense to refer not only to discrete items of solid matter but also to aggregated items of solid matter, and discrete or aggregated bubbles or drops of liquid material.
Gravity separation is concerned with the separation of particles on the basis of density, often requiring the hydrodynamic suppression of the effects of particle size. Various technologies have been developed to promote gravity separation, but all suffer from the effects of particle size variation within the feed. Ideally, in gravity separation the low density particles report as part of one flow stream, and the higher density particles report as another stream. In practice, however, this ideal result is not achieved. By way of example in a fluidized bed separator the higher density particles generally settle faster, but the very finest of the high density particles settle slowly, and join the lower density stream. Conversely, the very largest of the low density particles settle more rapidly and will appear with the denser particles. In a spirals separator the separation is more complex, however, again the separation only covers a limited size range.
Enhanced gravity separation methods utilize centrifugal forces to promote the separation of ultrafine particles, typically down to 0.010 mm. These devices operate according to the principles of solid-liquid fluidized beds. Through an increase in the so-called “g force” higher settling velocities and hence higher solids rates are achieved. With the higher “g force”, the intermediate regime of settling shifts to finer particles, which in turn reduces the dependence of the particle settling velocity on particle size for those finer particles. Hence, the centrifugal force suppresses the effects of particle size, in turn promoting gravity separation below 0.100 mm and often down to 0.01 mm.
The present invention is derived from a new and powerful separation mechanism, using closely spaced inclined channels. With closely spaced inclined channels the flow becomes laminar and the shear rate increases, producing inertial lift. Particles which settle within the intermediate flow regime, with a particle Reynolds number between about 1 and 500, elutriate on the basis of density, with particle size playing almost no role. Particles larger than about 0.100 mm thus separate on the basis of density. For binary systems involving a significant density difference between the particle species, complete separation of particles larger than about 0.040 mm is possible. This mechanism has been used in a Reflux Classifier of the type described in our International Patent Application PCT/AU00/00058 modified with the closely spaced inclined channels separated by, for example, a distance of 1.77 mm. The inclined channels were 1.0 m long.